linux-kernel - Re: Race condition in TTY ldisc layer in 2.6.33

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Date:	Thu, 11 Mar 2010 11:21:11 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
cc:	Greg KH <greg@...ah.com>, <linux-kernel@...r.kernel.org>,
	<linux-usb@...r.kernel.org>
Subject: Re: Race condition in TTY ldisc layer in 2.6.33

On Thu, 11 Mar 2010, [iso-8859-2] Micha³ Miros³aw wrote:

> On Thu, Mar 11, 2010 at 04:50:15PM +0100, Micha³ Miros³aw wrote:
> > On Thu, Mar 11, 2010 at 05:50:09AM -0800, Greg KH wrote:
> > > On Thu, Mar 11, 2010 at 02:32:22PM +0100, Micha? Miros?aw wrote:
> > > > Looks like there's some race contition in switching ldiscs on USB serial
> > > > ports. The following warnings trigger sometimes after killing and restarting
> > > > process that changes ldisc and waits forever. In case you want to look at
> > > > the code, there it is: http://rere.qmqm.pl/~mirq/sermmc/
> > > If you apply git commit 638b9648ab51c9c549ff5735d3de519ef6199df3 to the
> > > 2.6.33 kernel, does it solve the issue for you?
> > I'm running with the patch now. After couple cycles of starting and killing
> > the ldisc-setting process I get no warnings. I'll get back to you when/if
> > I encounter them again.
> 
> Hah. Just seconds after I sent that mail I hit this again. The stack traces
> are exactly the same (except for different starting address of
> flush_to_ldisc()).
> 
> The warnings are triggered by:
> 
> 	struct usb_serial_port *port = tty->driver_data;
> 	...
> 	/* count is managed under the mutex lock for the tty so cannot
> 	   drop to zero until after the last close completes */
> 	WARN_ON(!port->port.count);
> 
> BTW, Couple of minutes earlier I got this message:
> 
> [  201.629616] cp210x ttyUSB0: usb_serial_generic_resubmit_read_urb - failed resubmitting read urb, error -1
> 
> But after that I disconnected and reconnected the device, so this is probably
> not relevant here.

It looks like Greg gave you the wrong commit ID.  You should use this
one: 49d3380e3f1297ff7bdc700c0a7fe6c3a036b3ab.  It removes those
WARN_ON statements entirely.

Actually, you should use both commits.

Alan Stern

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